    Patent document 1 Japanese Patent Laid-Open No. 2003-318987 bulletin    Patent document 2 Japanese Patent Laid-Open No. 2003-234784 bulletin
The Denial of Service attacks, widely observed in recent years, transmits a large volume of spurious communication packets targeted at specific enterprises and organizations to disrupt their services. Such (D)DoS attacks obstruct other regular business communications by consuming a major portion of the link bandwidth and/or the processing resources of the server. This obstructs the routine work, hinders users from accessing services, the poor response time results in longer connection hours which in turn leads to increase in invoice amounts.
A DoS attack is a method of sending a large number of unwanted packets to a target equipment. When the volume of incoming packets is more than the processing capacity of the target equipment, the equipment will not be able handle the regular communication packets from regular users. In this case the equipment is effectively disabled; users cannot access its services.
In this case, one can think of a service which monitors and records the value of the Source address field in the header part of the offending packet, refuses to receive any further packets from these offending addresses (DDoS attack sources) and also sends a termination signal to the source of the offending packet.
However, this type of a defense against DDoS becomes a problem when the attack comes from several sources
Attempting to sent a “refusal to receive” to the source of each of the (D)Dos packet sources would exhaust the resources of the local host and network.
Thus by sending packets with randomly spoofed source addresses, an effective (D)DoS attack is achieved; it cannot be filtered and cannot be blocked.
In Patent-1 for example, if a very large volume of mail has been received, the To address (destination address) included in the header part of the E-mail is overwritten to allow load distribution among mail servers.
Patent document 2 adopts the procedure that only emails which have a proper reply address are judged to be valid. So all mails will be checked for valid reply addresses. The faked sources will not be able to receive replies properly and will be rejected.